Light mono-olefins, such as ethylene, propylene, and butylene, serve as feeds for the production of numerous chemicals including hydrocarbon-based polymers such as polyethylene, polypropylene, and the like. Light mono-olefins are generally prepared from a hydrocarbon feed, which may be derived from petroleum or renewable feedstocks. Hydrocarbon feeds generally include a mixture of paraffins and other hydrocarbons such as dienes, acetylenes, and the like, as well as various impurities such as sulfur-, mercury-, arsenic-, nitrogen-, and/or oxygen-containing compounds.
Processes for preparing mono-olefins from hydrocarbon feed are well known in the art and generally involve a combination of separation steps and reaction steps to optimize yield of desired mono-olefins. In particular, processes for preparing mono-olefins generally involve separating the hydrocarbon feed in one or more fractionation columns, thereby separating fractions of higher vapor pressure from fractions of lower vapor pressure, with progressive fractions being separated, from those having higher vapor pressures to those having lower vapor pressures, in successive fractionation columns that operate in series. The separated fractions are then subject to cracking or dehydrogenation reactions, depending upon the content of the feed and particular process design, to yield mono-olefins. The cracking and dehydrogenation reactions also produce dienes and/or acetylenes which, if recycled back to the cracking or dehydrogenation reactions in a recycle stream, may result in deposition of coke upon conversion catalysts, such as dehydrogenation catalysts, used in the cracking and dehydrogenation reactions. Deposition of coke on the conversion catalysts used in the cracking and dehydrogenation reactions is undesirable and, therefore, the separated fractions are generally subject to a selective hydrogenation reaction to convert dienes and/or acetylenes from the separated fraction into corresponding mono-olefins after the cracking or dehydrogenation reactions.
After the separated fractions are subject to cracking or dehydrogenation reactions and selective hydrogenation, mono-olefins are then separated therefrom, with unreacted paraffins and other components recycled back to the fractionation columns. To separate the mono-olefins, the product stream to be separated must be in liquid form. However, the cracking and dehydrogenation reactions occur under harsh conditions, with the product stream generally in vapor form. As such, the product stream from the cracking or dehydrogenation reactions requires cooling and condensation prior to separation of the mono-olefins.
Various apparatuses and devices are known for cooling and condensing vaporized product streams from the cracking or dehydrogenation reactions. Air and water-cooled apparatuses are generally employed to cool the vaporized product streams prior to compressing the vaporized product streams in a compressor. Such air and water-cooled apparatuses generally exhibit a pressure drop of about 27.5 kilopascals (kPa) across the air and water-cooled apparatuses, thereby requiring the cracking or dehydrogenation reaction to occur at pressures of about 35.5 kPa to maintain flow of the vaporized product streams through the air and water-cooled apparatuses and into the compressor.
The vaporized product stream is then compressed to sufficiently high pressures (generally in the range of from 700 to 1400 kPa-g (100-200 psig)) to allow for cooling and condensation using air or water cooling after compressing. In order to minimize the energy associated with compression of the vaporized product stream, it is common to cool the vaporized product stream to about 38° C. using air or water cooled exchangers before entering the compressor. However, the associated pressure drop of the vaporized product stream through such air or water cooled exchangers, typically from 10 to 20 kPa, forces the cracking or dehydrogenation reaction to occur at higher pressures, which are not as favorable for the reactions. In particular, lower reaction pressures are generally desired for cracking and dehydrogenation reactions, with higher paraffin to olefin conversion obtainable due to favorable dehydrogenation equilibriums at lower pressures.
In an industrialized setting, it may be desirable to employ contact cooling to cool the vaporized product streams with a cool impurity-containing liquid hydrocarbon stream, such as a light cycle oil stream, after the cracking or dehydrogenation reactions due to ready availability of such cool impurity-containing liquid hydrocarbon streams. Contact cooling may be desirable due to low pressure drop of about 3.5 kPa that is generally attendant across the contact cooling apparatuses, thereby enabling the cracking and hydrogenation reactions to occur at lower pressures that enable maximized paraffin to olefin conversion to be attained. However, selective hydrogenation catalysts that include noble metals are sensitive to many impurities that are prevalent in the impurity-containing liquid hydrocarbon streams, such as light cycle oil, and may experience deactivation or reversible inhibition that requires catalyst recovery when exposed to such impurities. As such, contact cooling of the vaporized product streams using a cool liquid hydrocarbon stream is often unfeasible or requires non-traditional selective hydrogenation catalysts, which are free from noble metals, to be employed in the selective hydrogenation reaction.
Accordingly, it is desirable to provide processes for preparing mono-olefins that enable contact cooling using an impurity-containing liquid hydrocarbon stream to be employed in conjunction with selective hydrogenation of dienes and/or acetylenes, even while employing selective hydrogenation catalysts that include noble metals. It is also desirable to provide apparatuses configured to support processes for preparing mono-olefins that include a selective hydrogenation stage and a contact cooling stage in which an impurity-containing liquid hydrocarbon stream can be employed in the contact cooling stage with minimal effect on selective hydrogenation catalysts, even when the selective hydrogenation catalysts including noble metals are employed. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.